package org.eclipse.aether.connector.basic;
   
   /*
    * Licensed to the Apache Software Foundation (ASF) under one
    * or more contributor license agreements.  See the NOTICE file
    * distributed with this work for additional information
    * regarding copyright ownership.  The ASF licenses this file
    * to you under the Apache License, Version 2.0 (the
    * "License"); you may not use this file except in compliance
   * with the License.  You may obtain a copy of the License at
   *
   *  http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing,
   * software distributed under the License is distributed on an
   * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
   * KIND, either express or implied.  See the License for the
   * specific language governing permissions and limitations
   * under the License.
   */
  
  import org.slf4j.Logger;
  import org.slf4j.LoggerFactory;
  
  import java.io.Closeable;
  import java.io.File;
  import java.io.IOException;
  import java.io.RandomAccessFile;
  import java.nio.channels.Channel;
  import java.nio.channels.FileLock;
  import java.nio.channels.OverlappingFileLockException;
  import java.util.UUID;
  import java.util.concurrent.atomic.AtomicBoolean;
  
  /**
   * A partially downloaded file with optional support for resume. If resume is enabled, a well-known location is used for
   * the partial file in combination with a lock file to prevent concurrent requests from corrupting it (and wasting
   * network bandwith). Otherwise, a (non-locked) unique temporary file is used.
   */
  final class PartialFile
      implements Closeable
  {
  
      static final String EXT_PART = ".part";
  
      static final String EXT_LOCK = ".lock";
  
      interface RemoteAccessChecker
      {
  
          void checkRemoteAccess()
              throws Exception;
  
      }
  
      static class LockFile
      {
  
          private final File lockFile;
  
          private final FileLock lock;
  
          private final AtomicBoolean concurrent;
  
          LockFile( File partFile, int requestTimeout, RemoteAccessChecker checker )
              throws Exception
          {
              lockFile = new File( partFile.getPath() + EXT_LOCK );
              concurrent = new AtomicBoolean( false );
              lock = lock( lockFile, partFile, requestTimeout, checker, concurrent );
          }
  
          private static FileLock lock( File lockFile, File partFile, int requestTimeout, RemoteAccessChecker checker,
                                        AtomicBoolean concurrent )
              throws Exception
          {
              boolean interrupted = false;
              try
              {
                  for ( long lastLength = -1L, lastTime = 0L;; )
                  {
                      FileLock lock = tryLock( lockFile );
                      if ( lock != null )
                      {
                          return lock;
                      }
  
                      long currentLength = partFile.length();
                      long currentTime = System.currentTimeMillis();
                      if ( currentLength != lastLength )
                      {
                          if ( lastLength < 0L )
                          {
                              concurrent.set( true );
                              /*
                               * NOTE: We're going with the optimistic assumption that the other thread is downloading the
                               * file from an equivalent repository. As a bare minimum, ensure the repository we are given
                               * at least knows about the file and is accessible to us.
                               */
                             checker.checkRemoteAccess();
                             LOGGER.debug( "Concurrent download of {} in progress, awaiting completion", partFile );
                         }
                         lastLength = currentLength;
                         lastTime = currentTime;
                     }
                     else if ( requestTimeout > 0 && currentTime - lastTime > Math.max( requestTimeout, 3 * 1000 ) )
                     {
                         throw new IOException( "Timeout while waiting for concurrent download of " + partFile
                                                    + " to progress" );
                     }
 
                     try
                     {
                         Thread.sleep( 100 );
                     }
                     catch ( InterruptedException e )
                     {
                         interrupted = true;
                     }
                 }
             }
             finally
             {
                 if ( interrupted )
                 {
                     Thread.currentThread().interrupt();
                 }
             }
         }
 
         private static FileLock tryLock( File lockFile )
             throws IOException
         {
             RandomAccessFile raf = null;
             FileLock lock = null;
             try
             {
                 raf = new RandomAccessFile( lockFile, "rw" );
                 lock = raf.getChannel().tryLock( 0, 1, false );
 
                 if ( lock == null )
                 {
                     raf.close();
                     raf = null;
                 }
             }
             catch ( OverlappingFileLockException e )
             {
                 close( raf );
                 raf = null;
                 lock = null;
             }
             catch ( RuntimeException | IOException e )
             {
                 close( raf );
                 raf = null;
                 if ( !lockFile.delete() )
                 {
                     lockFile.deleteOnExit();
                 }
                 throw e;
             }
             finally
             {
                 try
                 {
                     if ( lock == null && raf != null )
                     {
                         raf.close();
                     }
                 }
                 catch ( final IOException e )
                 {
                     // Suppressed due to an exception already thrown in the try block.
                 }
             }
 
             return lock;
         }
 
         private static void close( Closeable file )
         {
             try
             {
                 if ( file != null )
                 {
                     file.close();
                 }
             }
             catch ( IOException e )
             {
                 // Suppressed.
             }
         }
 
         public boolean isConcurrent()
         {
             return concurrent.get();
         }
 
         public void close() throws IOException
         {
             Channel channel = null;
             try
             {
                 channel = lock.channel();
                 lock.release();
                 channel.close();
                 channel = null;
             }
             finally
             {
                 try
                 {
                     if ( channel != null )
                     {
                         channel.close();
                     }
                 }
                 catch ( final IOException e )
                 {
                     // Suppressed due to an exception already thrown in the try block.
                 }
                 finally
                 {
                     if ( !lockFile.delete() )
                     {
                         lockFile.deleteOnExit();
                     }
                 }
             }
         }
 
         @Override
         public String toString()
         {
             return lockFile + " - " + lock.isValid();
         }
 
     }
 
     static class Factory
     {
 
         private final boolean resume;
 
         private final long resumeThreshold;
 
         private final int requestTimeout;
 
         private static final Logger LOGGER = LoggerFactory.getLogger( Factory.class );
 
         Factory( boolean resume, long resumeThreshold, int requestTimeout )
         {
             this.resume = resume;
             this.resumeThreshold = resumeThreshold;
             this.requestTimeout = requestTimeout;
         }
 
         public PartialFile newInstance( File dstFile, RemoteAccessChecker checker )
             throws Exception
         {
             if ( resume )
             {
                 File partFile = new File( dstFile.getPath() + EXT_PART );
 
                 long reqTimestamp = System.currentTimeMillis();
                 LockFile lockFile = new LockFile( partFile, requestTimeout, checker );
                 if ( lockFile.isConcurrent() && dstFile.lastModified() >= reqTimestamp - 100L )
                 {
                     lockFile.close();
                     return null;
                 }
                 try
                 {
                     if ( !partFile.createNewFile() && !partFile.isFile() )
                     {
                         throw new IOException( partFile.exists() ? "Path exists but is not a file" : "Unknown error" );
                     }
                     return new PartialFile( partFile, lockFile, resumeThreshold );
                 }
                 catch ( IOException e )
                 {
                     lockFile.close();
                     LOGGER.debug( "Cannot create resumable file {}", partFile.getAbsolutePath(), e );
                     // fall through and try non-resumable/temporary file location
                 }
             }
 
             File tempFile =
                 File.createTempFile( dstFile.getName() + '-' + UUID.randomUUID().toString().replace( "-", "" ), ".tmp",
                                      dstFile.getParentFile() );
             return new PartialFile( tempFile );
         }
 
     }
 
     private final File partFile;
 
     private final LockFile lockFile;
 
     private final long threshold;
 
     private static final Logger LOGGER = LoggerFactory.getLogger( PartialFile.class );
 
     private PartialFile( File partFile )
     {
         this( partFile, null, 0L );
     }
 
     private PartialFile( File partFile, LockFile lockFile, long threshold )
     {
         this.partFile = partFile;
         this.lockFile = lockFile;
         this.threshold = threshold;
     }
 
     public File getFile()
     {
         return partFile;
     }
 
     public boolean isResume()
     {
         return lockFile != null && partFile.length() >= threshold;
     }
 
     public void close() throws IOException
     {
         if ( partFile.exists() && !isResume() )
         {
             if ( !partFile.delete() && partFile.exists() )
             {
                 LOGGER.debug( "Could not delete temporary file {}", partFile );
             }
         }
         if ( lockFile != null )
         {
             lockFile.close();
         }
     }
 
     @Override
     public String toString()
     {
         return String.valueOf( getFile() );
     }
 
 }